Hypoxia-Inducible Factors Activate Breast Cancer Metastatic Genes

Image: In low oxygen conditions, breast cancer cells form structures that facilitate movement, such as filaments that allow the cell to contract (green) and cellular \"hands\" that grab surfaces to pull the cell along (red) (Photo courtesy of Dr. Daniele Gilkes, Johns Hopkins University).

Factors related to hypoxia, the condition of low oxygen content that characterizes the microenvironment surrounding breast tumors, have been linked to the activation of genes that encode proteins that transform benign breast cancer cells into mobile and invasive metastatic cells.

Hypoxia-inducible factors (HIFs) are transcription factors that respond to changes in available oxygen in the cellular environment, specifically, to decreases in oxygen, or hypoxia. HIFs promote the activation of genes involved in cancer initiation, progression, and metastases. Hypoxia has been shown to enhance the invasiveness and metastatic potential of tumor cells by regulating the genes involved in the breakdown of the ECM (extracellular matrix) as well as genes that control motility and adhesion of tumor cells. HIF activity is upregulated by mutated RAS, a member of the KRAS family of oncogenes, and BRAF (v-raf murine sarcoma viral oncogene homolog B1) as well as loss-of-function mutations of the PTEN gene. PTEN (phosphatase and tensin homolog), which is missing in 60%–70% of metastatic cancers in humans, is the name of a phospholipid phosphatase protein, and gene that encodes it. The PTEN gene acts as a tumor suppressor gene thanks to the role of its protein product in regulation of the cycle of cell division, preventing cells from growing and dividing too rapidly.

Investigators at Johns Hopkins University (Baltimore, MD, USA) reported in the December 9, 2013, online edition of the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) that HIFs triggered metastasis by increasing the activity of the proteins RhoA and ROCK1.

RhoA (Ras homolog gene family, member A) is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. This protein is essential for the signaling function of the Rho GTPase complex. Previous studies have shown that in breast cancer increased RhoA activity stimulated cancer cell invasiveness and spreading, while RhoA deficiency suppressed cancer growth and progression. Rho kinase 1 (ROCK1) is a kinase that regulates myosin light-chain activity, leading to actin-myosin contraction, which is the basis for cell movement.

The investiagors found that the RhoA-bound ROCK1 complex phosphorylated myosin light chain (MLC), which was required for actin-myosin contractility. RhoA also activated focal adhesion kinase (FAK) signaling. Increased activity of these two proteins led to cell and matrix contraction, focal adhesion formation, and motility through the phosphorylation of MLC and FAK.

“As tumor cells multiply, the interior of the tumor begins to run out of oxygen because it is not being fed by blood vessels,” said senior author Dr. Gregg Semenza, professor of medicine at Johns Hopkins University. “The lack of oxygen activates the hypoxia-inducible factors, which are master control proteins that switch on many genes that help cells adapt to the scarcity of oxygen."

“High levels of RhoA and ROCK1 were known to worsen outcomes for breast cancer patients by endowing cancer cells with the ability to move, but the trigger for their production was a mystery,” said Dr. Semenza. “We now know that the production of these proteins increases dramatically when breast cancer cells are exposed to low oxygen conditions.”
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